3. Sedimentary Rocks: Layers of Time

Sedimentary rock is the great record-keeper of Earth's history. It forms at or near the surface, building up in layers over long stretches of time — and those layers can trap fossils.

The process (sometimes called lithification) goes step by step:

1. Weathering breaks existing rock into small pieces (sediment).
2. Erosion carries the sediment away, usually by water, wind, or ice.
3. Deposition drops the sediment in flat layers, often in lakes or the sea.
4. Compaction & cementation press and glue the layers into solid rock.

There are three main kinds, by what the sediment is:

• Clastic — bits of other rock cemented together (sandstone, shale, conglomerate).
• Chemical — minerals left behind when water evaporates (rock salt, some limestone).
• Organic — built from the remains of living things (coal from plants; much limestone from shells).

Because layers stack up over time, in undisturbed rock the oldest layer is on the bottom and the youngest on top — the law of superposition, a key tool for reading Earth's past.

⚠️ Common misconception: fossils are found almost only in sedimentary rock. The intense heat that forms igneous and metamorphic rock usually destroys any remains.

💡 Tip: think 'sediment → settles in layers.' Flat layers + fossils = sedimentary; bottom layer = oldest.